I use cheap carbon steel hack saw blades. They can be found in packs if 2-5 for $1-$2. I cut them to about 2.25" segments. Next I drill a hole in one end (if it is a non-end piece) large enough for a piece of 550 cord (1/8th" or so). Next I rub each piece on a sanding block and remove the paint from the edges on both sides. Nothing crazy or precise there. Finally, I attach a cylindrical grinding stone to my RotoZip tool, round the sharp corners slightly, and put a squared, flat edge opposite the saw teeth and on the end opposite of the lanyard hole. I leave the teeth as they are.
I have BSA Hot Sparks and Doan's mag blocks scattered around, and all have carbon steel hack saw blade strikers/scrapers. The Hot Psark attached to my EDC Leatherman gets used weekly for starting the grill, outdoor fireplace, etc., and has been a consitently strong sparker.
To address your one-pice idea, I had thought of that, too. However, knife + ferro rod = 2 pieces anyways. I just figured I might as well attach a dedicated scraper to the rod or block, and the knife can be used as a backup scraper if need be. The hack saw blade piece and the 550 cord don't weigh much, and I usually girth hitch the fire starter to the knife or knife sheath to help insure that I have one handy. The BSA Hot Spark is small enough (and cheap enough!) that aside from the one on my person, I usually have one in whatever gear I have with me. I am at work right now, in my office. There is a Hot Spark in my pocket attached to my Leatherman, and a one in my little back pack that I use to ferry lunch and stuff to work.